Display screen or portion thereof with a transitional graphical user interface



FIG. 1 is a front view of a display screen or portion thereof with a transitional graphical user interface showing our new design;

FIG. 2 is a front view of a display screen or portion thereof with a transitional graphical user interface showing our new design; and,

FIG. 3 is a front view of a display screen or portion thereof with a transitional graphical user interface showing our new design.

The broken line showing of the display screen is for the purpose of illustrating environment and forms no part of the claimed design. The broken line showing of elements on the display screen illustrate portions of the graphical user interface and form no part of the claimed design.

The appearance of the transitional image sequentially transitions between the images shown in FIGS. 1-3. The process or period in which one image transitions to another image forms no part of the claimed design. 

CLAIM The ornamental design for a display screen or portion thereof with a transitional graphical user interface, substantially as shown and described. 